Cooling arrangement for electro-optical character generator

ABSTRACT

A character generator for a printer or copier which illuminates a surface of a photoconductor is provided with a cooling device, wherein a line of LEDs is mounted on a bearing surface of a hollow, thin walled profile that runs along the direction of the LED row. The hollow interior is filled with a liquid. On the other side of the hollow profile from the row of LEDs is a heat dissipating structure, such as a finned heat sink.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electro-optical character generator for theillumination of the surface of a photoconductor, in particular of aphotoconductor in a high-performance printer, having a heat collector onwhose bearer surface--facing the photoconductor--there is arranged amultiplicity of light-emitting elements, arranged in a row, for theillumination of the surface of the photoconductor, which are connectedin heat-conducting fashion with the heat collector, and in which thereis fashioned a hollow space running in the direction of the row, whichis filled with a liquid whose quotient of supplied quantity of heat andchange of temperature per unit of volume is greater than or equal to 2.5kJ/dm³ K, and having a cooling means connected to the heat collector,for giving off the quantity of heat received by the heat collector tothe environment.

2. Description of the Related Art

Electro-optical character generators are used primarily in copiermachines and printers. By means of illumination on the surface of aphotoconductor, they produce a latent charge image corresponding to thelater print image, which image is colored in with toner particles. Thecolored-in charge image is subsequently transferred to a recordingmedium using a corona means, and is fixed on the surface thereof in afixing means. Known character generators have a heat collector thatextends in the longitudinal direction of the photoconductor and servessimultaneously as a bearer, on whose one bearing surface, facing thephotoconductor, there is arranged a multiplicity of light-emittingelements in a row next to one another, as well as an optical meansfastened to the heat collector, which sharply images the light pointsproduced by the light-emitting elements on the surface of thephotoconductor.

In addition, the character generator is equipped with a controlelectronics that drives the individual light-emitting elementsindependently of one another, by means of a multiplicity of integratedcircuits (ICs), in such a way that the quantity of light respectivelyemitted by the elements is adjustable, and different charge states, andthus different grey gradations or, respectively, color gradations in thelater print image, can be realized on the surface of the photoconductor.

As light-emitting elements, light-emitting diodes (called LEDs in thefollowing) are suitable, particularly at an image point density of 600dpi (dots per inch) and higher, which are fastened in groups of e.g. 128LEDs on a common chip, known as LED arrays, in a line next to oneanother. Dependent on the width of the photoconductor, several such LEDarrays are fastened next to one another on the bearer surface of theheat collector, the surface running in the longitudinal direction of thephotoconductor, and are driven via the control electronics, which ifwarranted is also connected fixedly with the bearer.

In this LED array, power losses of up to 6 W per LED array can occur, sothat in a high-performance printer that has for example a print width of30 inches and uses approximately 140 such LED arrays for theillumination of the surface of the photoconductor, power losses ofapproximately 850 W occur. The heat quantity that arises in this way hasto be removed, because the surface temperature of each LED may notexceed 50° C. during operation. This is because if the surfacetemperature of the LED is higher, the quantity of light produced by theLEDs decreases, so that the surface of the photoconductor can no longerbe illuminated by the LEDs with the same high quality.

For this reason, the light-emitting elements are connected inheat-conducting fashion with the heat collector, which collects thequantity of heat produced by the elements in order to keep the surfacetemperature of the elements below a critical temperature value, beyondwhich, as explained above, a high-quality illumination of thephotoconductor is no longer possible. By means of a cooling meansconnected with the heat collector, the quantity of heat stored by theheat collector is given off to the surrounding environment.

It is known to adapt the actual heat capacity of the heat collector,which results from the weight-specific heat capacity of the materialmultiplied by the mass of the material used, to the quantity of heatproduced by the light-emitting elements as lost power in such a way thatthis quantity of heat can be removed quickly, in order to prevent heatblockages and the resulting overheatings of the light-emitting elements.For this purpose, heat collectors are used that consist of a metalmaterial with a high weight-specific heat capacity, such as aluminum,copper or the like. The level of the actual heat capacity is determinedby the mass of the heat collector used.

Drawn or extruded full profiles made of the correspondingly suitablemetal materials are used as heat collectors, which have the requiredmass, and thereby heat capacity, to be able to store the quantities ofheat that occur. However, these heat collectors have the disadvantagethat, despite the high section modulus of the full profile, due to theirhigh intrinsic weight they bend so strongly that a uniform sharp imagingof the image points produced by the light-emitting elements on thesurface of the photoconductor by the optical means is no longerpossible. This problem occurs in particular in high-performance printersand copiers with broad photoconductors. The bending of the bearer of ahigh-performance printer that can print two paper webs with DIN A4format or letter size format at the same time can amount toapproximately 40 or 50 μm. During the imaging of the image point, whosediameter is approximately 60 μm, the optical means thereby produces animaging error of 3 to 5 μm, so that a sharp setting of the image pointsover the entire width of the surface of the photoconductor becomesimpossible.

In order to increase further the cooling power of the heat collector, itis further known to fashion cooling channels in the heat collector,through which there flows a liquid that removes the heat.

Thus, EP-0 629 508 A2 specifies a character generator withlight-emitting elements, on whose lower side a heat collector isfastened. A U-shaped channel is fashioned in the heat collector, whichis connected with an external cooling means and forms a cooling circuittherewith, through which water flows as a cooling liquid.

In this known character generator, there is the problem that the heatcollector has to be made of a material that is able to store theoccurrent quantities of heat until they are transported away by thecooling liquid. The actual heat capacity of the heat collector resultsfrom the weight-specific heat capacity of the material multiplied by themass of the material used. So that the heat collector possesses asufficiently high heat capacity, it must be fashioned correspondinglyheavily. However, this has the disadvantage that, due to its highintrinsic weight, the heat collector bends so strongly that a uniformsharp imaging of the image points produced by the light-emittingelements on the surface of the photoconductor is no longer possible. Inaddition, there is the problem that the heat collector is non-uniformlycooled by the water flowing through the channel as a cooling liquid. Thelight-emitting elements arranged near the entry opening of the channelare cooled more strongly by the cold water flowing in than are thelight-emitting elements provided at the other end of the heat collector.There thus results a non-uniform temperature distribution over thelength of the heat collector, which reduces the print quality of thecharacter generator.

From JP-A 63 168 372, a character generator is known on whose upper sideis fastened a transparent covering that protects the light-emittingelements. On the lower side of the character generator there is fasteneda heat collector in which a channel system is fashioned. The hollowspace formed by the covering with the upper side of the charactergenerator is connected with the channel system in the heat collector.For the cooling of the light-emitting elements, a cooling liquidconsisting of water and alcohol flows through the hollow space and thechannel system.

SUMMARY OF THE INVENTION

The object of the invention is to provide a character generator ofsimple construction that ensures a uniform heat distribution over theentire length of the character generator, with a low degree of bending.

This object and others are achieved for an electro-optical charactergenerator of the type named above in that the hollow space in the heatcollector is fashioned by a closed thin-walled hollow profile that runsin the direction of the row and is closed at its two ends. Advantageousdevelopments result from the improvements including that the hollowprofile is fashioned as a U-profile that has one base on whose upperside--facing the photoconductor--the bearer surface is fashioned, andthat has two limbs that are at least approximately equal in length andthat protrude from the lower side--facing away from the upper side--ofthe base, and in that the U-profile is closed via a base plateconnecting the limbs with one another, in such a way that the hollowspace extending in the direction of the row is fashioned, which hollowspace is respectively closed at each of its two open ends by aterminating plate. The invention is further characterized in that aprojection, raised from the base and extending in the direction of therow, is formed essentially in the center of the upper side of the base,which projection forms the bearer surface. The base plate may befashioned in one piece with the limbs of the U-profile. In addition, thebase plate may be connected in liquid-tight fashion with the limbs ofthe U-profile by means of joining, preferably by hard soldering. Theliquid within the space has a quotient within a range from 3.0 kJ/dm³ Kto 4.5 kJ/dm³ K. In one embodiment, the liquid is water.

The cooling means is a cooling grid with a bearer plate fastened inheat-conducting fashion to the hollow profile, from whose plate sidefacing away from the hollow profile there protrude several cooling ribsarranged in parallel with a spacing from one another. Alternately, thecooling means is a cooling grid with a bearer plate fastened inheat-conducting fashion to the hollow profile, from whose plate sidefacing away from the hollow profile there protrude several cooling ribsarranged in parallel with a spacing from one another. In a furtherembodiment, the cooling means is a cooling aggregate having a pump andhaving a heat exchanger, which aggregate forms a cooling circuit withthe hollow profile via a supply line and a drain, through which circuitthe liquid circulates, in order to give off to the environment, via theheat exchanger, the quantity of heat collected in the hollow profile.

In the invention, the rigidity of the heat collector is determined bythe hollow profile, while the level of the heat capacity of the heatcollector depends on the liquid. The heat capacity of the heat collectoris determined by the volume-specific heat capacity of the liquid, whichis defined as the quotient of the supplied quantity of heat in kJ andthe change of temperature in K in relation to a volume unit in dm³, andthe volume of liquid used.

By means of the selection of suitable liquids, such as glycerine, water,or the like, whose volume-specific heat capacity is greater than orequal to 2.5 kJ/dm³ K, the volume of the hollow space filled with theliquid, which space is surrounded by the thin-walled profile, can beminimized. The greater the volume-specific heat capacity of the liquidis, the smaller the volume of the hollow space can be fashioned, so thatthe constructive volume of the heat collector decreases correspondingly.The rigidity of the heat collector is decisively dependent on the hollowprofile, so that the bending of the heat collector can be minimized byoptimizing the cross-sectional shape and by a suitable selection ofmaterial, whereby imaging errors due to the bending of the heatcollector are reduced, and the character generator operates with ahigher imaging quality.

The hollow profile preferably has a cross-sectional shape whosegeometrical moment of inertia is large enough that the bearer surfacecan bend only far enough that the light points produced by thelight-emitting elements are still sharply imaged on the surface of thephotoconductor, i.e. are imaged uniformly over the entire lengththereof.

In a preferred embodiment of the character generator, the hollow profileis fashioned as a U-profile that has a base on whose upper side, facingthe photoconductor, the bearer surface is fashioned, and that has twolimbs that are at least approximately equal in length and that protrudefrom the lower side--facing away from the upper side--of the base. Thelimbs are connected with one another via a base plate, so that theU-profile is closed and the hollow space extending in the direction ofthe row is fashioned. The hollow space is respectively closed at each ofits two open ends via a terminating plate. Due to the fashioning of thehollow profile as a U-profile, the heat collector has a high geometricalmoment of inertia, by means of which the bending of the electro-opticalcharacter generator is minimized.

Preferably, a projection, raised from the base, that forms the bearersurface is formed on the upper side of the base, approximately in thecenter, and extends in the direction of the row. In this way,corresponding space for the control electronics remains free on bothsides of the projection on the upper side of the base, whereby aparticularly compact construction of the character generator ispossible.

The U-profile can be manufactured by drawing or extrusion, whereby thebase plate is fashioned in one piece with the limbs of the U-profile. Ina further embodiment, the base plate is connected in liquid-tightfashion with the limbs of the U-profile by means of joining, i.e. bymeans of manufacturing methods such as welding, soldering or gluing. Asa metal material, non-ferrous materials such as aluminum, copper or thelike, as well as alloys thereof, are suited. However, conventionalconstructional steels or alloyed steels can also be used for the hollowprofile of the heat collector.

As a liquid for the heat collector, a liquid is proposed whose namedquotient lies in a range from 3.0 to 5 kJ/dm³ K, preferably in a rangefrom 3.5 to 4.5 kJ/dm³ K, since in comparison with their volume-specificheat capacity these liquids have a low density, and the intrinsic weightof the liquid, due to which the heat collector additionally bends, islow. Proposed liquids include glycerin with a volume-specific heatcapacity of approximately 3.0 kJ/dm³ K, or water with a volume-specificheat capacity of approximately 4.2 kJ/dm³ K, because the density of thisliquid, compared with the density of suitable metal materials withsimilarly high volume-specific heat capacities, is however low.

As a cooling means, a cooling grid is proposed that has a bearer platefastened in heat-conductive fashion to the hollow profile; from theplate side--facing away from the hollow profile--of this bearer plate,several cooling ribs arranged at a spacing from one another protrude inperpendicular fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in more detail on the basisof the drawing.

FIG. 1 shows a perspective view of a segment of a character generator,and

FIG. 2 shows a sectioned front view of a heat collector used in thecharacter generator according to FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a perspective view of a segment of a character generator 10used in a high-performance printer. The character generator 10 has aheat collector 12 that serves as a bearer, which extends transverse tothe direction of motion of a rotating photoconductor drum 14 (shown indotted lines) of the printer. As shown in FIG. 2, the heat collector 12,made of copper, is fashioned as a U-profile 16. The U-profile 16 has abase 18 on whose upper side (facing the photoconductor, which is notshown), approximately in the center, a projection 20 raised from thebase 18 and extending in the longitudinal direction of the U-profile 16is formed. The upper side--facing the photoconductor drum 14--of theprojection 20 is microfinished, and serves as a bearer surface 22 forlight-emitting elements, as is explained below. The longitudinal edgesof the base 18 are bent around in such a way that two limbs 24 and 26,approximately equal in length and extending in the longitudinaldirection of the U-profile 16, are formed, which stand out approximatelyperpendicularly from the lower side--facing away from the photoconductordrum 14--of the base 18. The respective free end of each limb 24 and 26is bent outward at a right angle in such a way that on each limb 24 and26 a fastening edge 28 or, respectively, 30 is fashioned that extends inthe longitudinal direction of the U-profile 16 and runs approximatelyparallel to the upper side of the base 18. The lower surface--facingaway from the photoconductor drum 14--of each fastening edge 28 and 30is shaped flat, and serves as a support surface for a base plate 32 thatconnects the two limbs 24 and 26 with one another and extends over theentire length of the U-profile 16. The base plate 32 is hard-soldered inliquid-tight fashion with the fastening edges 28 and 30 of the U-profile16, so that a hollow space 34 extending in the longitudinal direction ofthe heat collector 12 is formed that is respectively sealed at its twoopen ends via a terminal plate (not shown). This hollow space 34 is, asshown in FIG. 1, filled with water 36, which serves as a heat-storingelement, as is explained below.

As shown in FIG. 1, in the center of the bearer surface 22 several LEDarrays 38 are connected in heat-conducting fashion with the heatcollector 12, whereby in this embodiment each LED array 38 bears 128light-emitting diodes (LEDs) arranged next to one another in a row. TheLED arrays 38 are likewise arranged next to one another, so that theirLEDs form an LED row 40 extending in the longitudinal direction of theheat collector 12, which row serves for the illumination of the surfaceof the photoconductor drum 14. Above the LED row 40, there isadditionally arranged an optical means that images the emission surfacesof the LEDs on the surface of the photoconductor drum 14, which means ishowever not shown for reasons of clarity.

On each side of the LED row 40, there is respectively arranged an IC row42 or, respectively, 44, extending in the longitudinal direction of theheat collector 12 and consisting of several ICs (integrated circuits),which are respectively connected in electrically conductive fashion withthe heat collector 12 and drive the individual LEDs of the LED row 40.On both sides of the projection 20, there is respectively arranged aconductor rail 46 or, respectively, 48 that has an L-shapedcross-section and extends in the longitudinal direction of the heatcollector 12, which is fixedly connected with the upper side of theU-profile 16 via an insulating layer 50 or, respectively, 52.Approximately at the level of the IC rows 42 and 44, each conductor rail46 and 48 bears a flat assembly 54 or, respectively, 56, connected inconductive fashion with these rails, on which assemblies interconnects(not shown) are respectively fashioned, which are connected with theindividual ICs of the IC rows 42 or, respectively, 44 via Bondconnections. On the lower side of the heat collector 12, a cooling grid58 is in addition fastened. The cooling grid 58 has a bearer plate 60that extends over the entire width of the base plate 32 and runs overthe entire length of the heat collector 12, which bearer plate isconnected in heat-conducting fashion with the base plate 32 and fromwhose plate side--facing away from the base plate 32--several coolingribs 62, running in the longitudinal direction of the heat collector 12and arranged in parallel with a spacing from one another, protrude inperpendicular fashion.

As soon as the printer begins print operation, the individual ICs of theIC rows 42 and 44 activate the various LEDs of the LED array 38, wherebythe surface of the photoconductor drum 14 is illuminated. Dependent onthe control data of the control electronics, the ICs vary the quantityof light given off by the LEDs, so that various charge states can berealized on the surface of the photoconductor drum 14, and various grayor, respectively, colored tones can thus be realized in the later printimage. Due to the constant switching on and off of the LEDs of the LEDarray 38, heat arises that has to be led off via the heat collector 12.For this purpose, the U-profile 16, made of copper, transfers the heatproduced by the LEDs to the water 36 located in the hollow space 34,which stores the heat and gradually transfers it to the cooling ribs 62of the cooling grid 58 via the base plate 32 and the bearer plate 60,the ribs giving the heat off to the environment.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

What is claimed is:
 1. Electro-optical character generator forilluminating a surface of a photoconductor, comprising:a heat collectorhaving a bearer surface facing the photoconductor; a multiplicity oflight-emitting elements arranged in a row for illumination of thesurface of the photoconductor, said light-emitting elements beingconnected in heat-conductive fashion with the heat collector; said heatcollector being fashioned with a hollow space running in a direction ofthe row; a liquid filled in said hollow space, said liquid having aquotient of supplied quantity of heat and change of temperature pervolume unit greater than or equal to 2.5 kJ/dm³ K; a cooling structureconnected with the heat collector for giving off a quantity of heatcollected by the heat collector to the environment; and the heatcollector which forms the hollow space being fashioned by a closedthin-walled hollow profile running in the direction of the row, saidhollow space being closed at two ends of said hollow space, said closedthin-walled hollow profile being of an elongated shape extending in adirection of the row and shaped to resist bending so as to maintain saidlight emitting elements in respective predetermined printing positionsrelative to one another.
 2. Electro-optical character generatoraccording to claim 1, wherein the hollow profile includinga U-profilethat has one base, said base having an upper side facing thephotoconductor, said upper side having the bearer surface, saidU-profile having two limbs that are at least approximately equal inlength and that protrude from a lower side facing away from the upperside of the base, and a base plate connecting the limbs with one anotherto close said U-profile in such a way that the hollow space extending inthe direction of the row is fashioned; a terminating plate mounted as ateach end of the hollow space to close the hollow space. 3.Electro-optical character generator according to claim 2, furthercomprising:a projection raised from the base and extending in thedirection of the row, said projection being formed essentially in acenter of an upper side of the base, said projection forming the bearersurface.
 4. Electro-optical character generator according to claim 2,wherein the base plate is fashioned in one piece with the limbs of theU-profile.
 5. Electro-optical character generator according to claim 2,wherein the base plate is connected in liquid-tight fashion with thelimbs of the U-profile by joining.
 6. Electro-optical charactergenerator according to claim 1, wherein said liquid has a quotient whichlies within a range from 3.0 kJ/dm³ K to 4.5 kJ/dm³ K. 7.Electro-optical character generator according to claim 1, wherein theliquid is water.
 8. Electro-optical character generator according toclaim 1, wherein said cooling structure is a cooling grid with a bearerplate fastened in heat-conducting fashion to the hollow profile, saidcooling structure having a plate side facing away from the hollowprofile and several cooling ribs protruding from said plate sidearranged in parallel with a spacing from one another.
 9. Electro-opticalcharacter generator according to claim 1, wherein said cooling structureis a cooling aggregate having a pump and having a heat exchanger, saidaggregate forming a cooling circuit with the hollow profile via a supplyline and a drain, the liquid circulates to give off to the environmentvia the heat exchanger heat collected in the hollow profile.